1. Field of the Invention
The present invention relates to enhancing plant growth with electrical lighting systems, and more particularly to enhancing plant growth by using optoelectronic devices, primarily light emitting diodes, in a continuous or pulsing mode, to produce sufficient irradiance to support specific photobiological reactions.
2. Background Information
Plants that contain the green pigment chlorophyll can transform the carbon dioxide in the atmosphere into sugars which are the primary nutrient materials for all living things. The chlorophyll molecule initiates this transformation by capturing light energy and converting it into chemical energy. This process is called photosynthesis. The generalized equation for the photosynthetic process is given as: EQU CO.sub.2 +H.sub.2 O+light.fwdarw.(CH.sub.2 O)+O.sub.2
The term (CH.sub.2 O) is an abbreviation for the basic chemical energy building block emanating from the photosynthetic process that is used for the synthesis of all plant components.
The radiant energy absorbed by chlorophyll is within that portion of the electromagnetic spectrum that enables humans to see. However, the absorption spectrum of chlorophyll is not the same as that for the human eye. Leaves of plants absorb violet, blue, orange, and red wavelengths most efficiently. The green and yellow wavelengths, to which the human eye is most sensitive, are reflected or transmitted and thus are not as important in the photosynthetic process (F. Zcheile and C. Comer. 1941. Botanical Gazette, 102:463.).
The interaction between chlorophyll and light quanta (photons) involves the sciences of molecular physics and spectrosopy. A few fundamental considerations of this interaction, as defined by these sciences, need to be discussed to provide some understanding of the behavior of photoactive plant pigments. These considerations are also pertinent to an understanding of the nature of this invention.
Light comes in discrete packets of energy termed quanta or photons. The energy in each photon is inversely proportional to the wavelength of the radiation. A photon of blue light has more energy than a photon of red light. Another fundamental consideration is that a light absorbing pigment, such as chlorophyll, can absorb only one photon at a time. A molecule of chlorophyll that has absorbed a photon is called an "excited" molecule. An "excited" molecule is ready to take part in a chemical reaction. Molecules in the normal or "unexcited" state do not take part in such chemical reactions. This excitation energy is the energy transferred in the photosynthetic process.
The lifetime of this excited state, however, is very short, in the pico and nanosecond range. Unless the excited molecule can transfer the excitation energy to an appropriate "acceptor" unit, the chance for a chemical reaction is lost. Other events, such as fluorescence or long wavelength radiation, will have dissipated the energy between the moment of excitation and the time of encounter with the "acceptor" unit if the time between such an encounter exceeds the lifetime of the excited state.
Eventual utilization of the "captured energy" in the photosynthetic process involves enzymatic reactions that include the reduction of carbon dioxide leading to the synthesis of sugars as the "first" products of photosynthesis. While the time constants of the primary photochemical process are very short, the time constants for the enzymatic reactions are relatively long; being on the order of micro- or milliseconds. It has been experimentally demonstrated that when light flashes of 100 microseconds duration follow each other too quickly, the light is not fully utilized (B. Kok. 1956. Photosynthesis in flashing light. Biochim. et Biophys. Acta, 21:245-258.). Extending the time of the dark period to about 20 milliseconds increased the relative "yield" per flash of light.
Although the capture of light energy for chemical purposes by the chlorophyll molecule makes photosynthesis the most important biological process, other light dependent reactions that regulate the metabolism of a plant are important. These photobiological reactions involve the plant responses identified as photomorphogenesis, phototropism, and photoperiodism. These photobiological reactions require very low levels of light as compared to photosynthesis. Also, the radiant energy wavelengths involved in these reactions differ from those most effective in the photosynthetic reaction.
Consequently, an electric light source for plants must not only provide an adequate intensity of light but also provide light of the proper spectral characteristics to meet the plant's requirements. A further important consideration of an electric light source pertains to the efficiency of conversion of the electricity to light with the desired spectral characteristics. The most commonly used electric light sources for plant growth are fluorescent and high pressure sodium lamps. These lamps have electrical conversion efficiencies ranging from 20 to 30 percent. Efficiencies significantly greater than this do not appear likely due to limitations inherent in the basic design of these types of lamps. Thus, a more effective electric light source for plant growth is desirable.